The invention relates to a method of access control in a communication system comprising a plurality of mutually independent subscriber stations, in which each subscriber station seizes a common duplex transmission channel for establishing a connection in accordance with a multiple-access method. Each subscriber station, to initiate a transmission from that station, inserts a signalling signal into a time slot of the duplex transmission channel and in response thereto is invited by a central unit of the communication system to identify itself fully.
Such an access method in a radio transmission system has already been proposed (P 31 33 347.8). The duplex transmission channel of the radio transmission system is divided into time slots in the forward and the return directions. Each time slot available for free access is subdivided into a plurality of short, so-called call slots (for example into eight call slots). The free access of a subscriber station to the duplex transmission channel is not effected with a complete identification, but by means of a signal of a significantly shorter duration, a so-called signalling short-duration signal, transmitted in one of the call slots. On receipt of a signalling short-duration signal, the central unit requests the relevant subscriber station to identify itself fully. The number of the call slot through which the signalling short-duration signal is transmitted is used in the central unit as an identification to distinguish between the subscriber stations which during a predetermined period have actually accessed the channel by means of their signalling short-duration signals. For transmitting the signalling short-duration signal which all the subscriber stations have in common, the subscriber stations can select one of the short call slots from a plurality of available call slots. The duration of a call slot is at least equal to the duration of the signalling short-duration signal, which duration is the same for all the subscriber stations. Selecting a call slot or the exact moment for transmitting the signalling short-duration signal in one of the call slots may occur at random.
When the central unit has recognized the signalling short-duration signal with adequate receiving quality in one of the call slots, it transmits a request for full identification to the subscriber station which is provisionally indicated by the number of the call slot. The request for full identification in an associated time slot excludes all the further subscriber stations from access.
As can be seen from the above-described embodiments of the proposed radio transmission system (P 31 33 347.8) the transmission of the signalling short-duration signals which are common to all the subscriber stations is effected in time-spaced, short call slots, the number of the selected call slot distinguishing the transmitting subscriber station from other subscriber stations. If two subscriber stations report in the same call slot, they cannot be distinguished from each other by the central unit.